1. Field of the Invention
The present invention relates to an apparatus for exposing peripheral portions of objects such as semiconductor wafers and, more specifically, to an apparatus for exposing the periphery of an object to remove photoresist applied on the periphery of the object.
2. Description of the Related Art
Recently, positive resist is being applied to a surface of a semiconductor wafer, a ceramic wafer or the like. The resist on the periphery of the wafer is removed by exposing the periphery of the wafer before or after exposing the desired patterns.
FIG. 1B is a side view of a conventional apparatus for exposing the periphery of a wafer and FIG. 1A is a plan view taken from the portion IA-IA of the apparatus for exposing the periphery of the wafer shown in FIG. 1B. Referring to FIGS. 1A and 1B, the conventional apparatus for exposing the periphery of a wafer comprises a table 11 for supporting a wafer W, a first motor M1 for rotating the table 11, and an illuminating apparatus Mv for exposing from above the periphery of the wafer W placed on the table 11. Referring to FIGS. 1A and 1B, the conventional apparatus for exposing periphery of a wafer has the below described structure. One illuminating apparatus Mv is provided spaced apart by a prescribed distance from the center of the wafer W, and the wafer W is rotated by the first motor M1, so that the periphery of the wafer is exposed. The periphery of the wafer W comprises an arc portion and a linear portion called an orientation flat. Exposure of the wafer periphery is carried out for the entire peripheral portion including the arc and the orientation flat portion.
When the wafer W is exposed by the conventional apparatus for exposing the periphery of the wafer, the arc portion can be exposed. However, at the linear portion, the distance of which from the center of the wafer W is shorter than that of the arc portion, the illuminating light illuminates outside the wafer. Therefore, the linear portion can not be exposed.
Therefore, the exposure of the linear portion has been carried out in the following manner.
(1) A plate cam having similar shape as the wafer W and which rotates in synchronization with the wafer W is provided, and a cam follower provided on the illuminating apparatus Mv is moved along the profile of the plate cam, so as to expose the linear portion by approximately the same width as the arc portion (plate cam method).
(2) Means for mechanically or optically detecting a wafer end portion (boundary between the arc portion and the linear portion) is provided. The detecting means detects the wafer end portion, and when the exposure portion moves from the arc to the linear portion, the wafer W is rotated with the distance from the center of the wafer W to the illuminating apparatus Mv adjusted so that the position of illumination by the illuminating apparatus Mv goes along the linear portion. Consequently, the linear portion can be exposed by approximately the same width as the arc portion (end portion detecting method).
The basic idea of the conventional methods is as follows. Namely, both the arc portion and the linear portion are exposed and the width of exposure of the arc portion and the linear portion are made approximately the same, with the wafer rotating at a constant speed. The conventional methods based on this idea have the following disadvantages.
(1) The illuminating apparatus Mv exposes a constant width from the outer periphery of the wafer W toward the center thereof. Therefore, referring to FIG. 2A, when the illuminating apparatus Mv illuminates the wafer end portions F1 and F2, the width of exposure L1 near the wafer end portions F1 and F2 becomes smaller than the width of exposure L0 of other portions, since the wafer end portions F1 and F2 have convex shape. This phenomenon becomes more conspicuous at the wafer linear portion Ws, and therefore, exposure by the same width can not be realized at the linear portion Ws.
(2) From the same reason as described above (1), when the illuminating apparatus Mv illuminates the wafer end portion F2 as shown in FIG. 2B, the length of exposure l1 in the inner side and the length of exposure l2 at the outer portion is very much different from each other (l1&lt;l2) in the same period of time. Since the length of exposure at respective points in the width direction shown by a in FIG. 2B are different from each other, the amount of exposure per unit area in the same period of time becomes different. Consequently, uniform exposure can not be realized.
(3) When the resist is applied on the surface of the wafer W, the wafer W is rotated in the direction shown by the arrow of FIG. 3. As shown in the figure, the resist tends to be collected on the end portion on the upstream side of the direction of rotation of the wafer. Consequently, the resist becomes thick in this region Wa (see hatched portion in the figure). Since the exposure is done with the wafer W rotated at a constant speed, the resist can not be sufficiently removed in this thick region Wa.
When the speed of rotation of the wafer W is made sufficiently slow in order to surely remove the resist of the thick region Wa, the amount of exposure is increased. However since, the total processing speed is reduced, the throughput is also reduced.
(4) The width of the thick region Wa (portion represented by b in FIG. 3) described above (3) generally becomes larger than the uniform width L0 of exposure. However, the width L0 of exposure is constant, the resist outside the exposure width L0 can not be removed, even if the thick region Wa should be entirely exposed.
(5) When the chip size is large, the pattern region P by the step exposure is considerably distant from the linear portion Ws as shown in FIG. 4. Unnecessary resist must be removed over regions as wide as possible from the linear portion Ws to the portion represented by the two dotted line. However, if the width of exposure L0 is constant in the arc portion Wc and in the linear portion Ws, the resist which is not in the exposure width L0 remains as it is.
(6) When the wafer W is treated in the succeeding processes, prescribed portions of the wafer W are gripped by clamping pawls k as shown in FIG. 5. In order to provide stable clamping, the regions Wk to be clamped must be exposed by the width larger than the uniform exposure width L0. The reason for this is that when the clamp region Wk is exposed by the uniform width L0, the resist at the portions to be clamped is not exposed, therefore the resist peels when it is gripped by the pawl k.